Creates a custom field for contacts. The name is auto-generated from the label as a slug.

Provide type (text, number, or date) or the existing contact_field_type_id UUID. If both are provided, they must identify the same type. UUIDs are the same for every account:

The value is immutable after creation and is not returned by any endpoint - responses carry the human-readable type (text, number, date) instead.

{
  "contact_field_type_id": "0abe5601-7738-43c8-858f-81911ecf89ee",
  "type": "text"
}